1. Field of the Invention
The present invention relates to a sheet feeder and, more particularly, to a sheet feeder equipped with a function of detecting double feed of sheets.
2. Description of the Related Art
There has been known a conventional sheet feeder relevant to the present invention including: a sheet supplying roller for feeding a sheet from a sheet supplying portion; a registration roller arranged at a predetermined interval from the sheet supplying roller; a wave transmitter provided on the way of a sheet feed path extending from the sheet supplying roller to the registration roller, for transmitting an ultrasonic wave; and a wave receiver for receiving the ultrasonic wave transmitted from the wave transmitter; wherein the wave transmitter transmits the ultrasonic wave to a sheet in the state in which the sheet is held between both of the sheet supplying roller and the registration roller, so that it is detected, by comparing an attenuation degree of the ultrasonic wave received by the wave receiver with a predetermined threshold, as to whether or not the sheets are doubly fed (see, for example, Japanese Unexamined Patent Publication No. 2005-162426).
In recent years, high-speed printing has been progressed in an image forming apparatus such as a copying machine or a printer.
For this purpose, a sheet feeder mounted on the image forming apparatus has been required to feed a sheet such as a document or a recording medium at a high speed.
However, there has been a high possibility of double feed of sheets in superimposition when a sheet is fed from a sheet supplying portion having sheets contained therein or stacked thereon onto a sheet feed path if the sheet is to be forcibly fed at a high speed.
Double feed of the sheets in superimposition is a major cause of a paper jam.
Even if no paper jam is caused, there occurs an inconvenience that an image different from an image to be read by an image reading unit is read in the doubly fed state, or that an image to be formed on a single sheet is formed over a plurality of superimposed sheets in an image forming unit.
As a consequence, the sheets cannot be sufficiently fed at a high speed only by increasing drive speed of feeding means such as a sheet supplying roller or a feeding roller. A function of securely detecting double feed of the sheets is required in the case where the sheets are fed in superimposition onto the sheet feed path.
Examples of means for detecting double feed of the sheets generally include an ultrasonic wave sensor disposed on the sheet feed path.
Such an ultrasonic wave sensor is constituted of a wave transmitter, which transmits an ultrasonic wave, and a wave receiver, which receives the ultrasonic wave transmitted from the wave transmitter so as to convert the ultrasonic wave into electric energy.
An attenuation degree of the ultrasonic wave received by the wave receiver can be compared with a predetermined threshold by utilizing the attenuation of the ultrasonic wave transmitted from the wave transmitter when the ultrasonic wave passes through the sheet, thereby detecting as to whether or not the sheets are doubly fed.
However, since the ultrasonic wave is transmitted via air as a medium, an output from the wave receiver is fluctuated if an amplitude, that is, a fluctuation of the sheet on the sheet feed path is large, thereby making it difficult to securely detect double feed.
In the above-described prior art, the ultrasonic wave is transmitted from the wave transmitter in the state in which the sheet is held between both of the sheet supplying roller and the registration roller, thus eliminating the fluctuation of the output from the wave receiver caused by the fluctuation of the sheet.
However, the sheet feeding is once stopped by the registration roller after the sheet is fed from the sheet supplying roller, and then, the sheet is held between both of the sheet supplying roller and the registration roller. Therefore, there is a limitation from the viewpoint of the sheet feeding at a high speed.
In addition, a design is limited such that the sheet supplying roller and the registration roller must be arranged with an interval shorter than the length of a sheet having a smallest size, and further, that the ultrasonic wave sensor must be interposed between the sheet supplying roller and the registration roller.